(1) Field of the Invention
The present invention relates to a process for the preparation of a light-diffusing methacrylic resin plate characterized in that the solar radiation transmittance is lower than the luminous transmittance thereof.
(2) Description of the Related Art
A process for preparing a polymer sheet having a pearl-like luster by polymerizing a mixture of an unsaturated monomer (hereinafter referred to as "monomer") composed mainly of methyl methacrylate and a leaf having a laminar structure, such as basic lead carbonate, a fish scale foil, bismuth trichloride or titanium dioxide-coated mica, is known. For example, there can be mentioned processes disclosed in Japanese Examined Patent Publications No. 31-9,355 and No. 33-294, that is, a process in which a polymerizable liquid material comprising a monomer and a leafy diffusing agent is cast in a plate-form casting mold, the leaf is appropriately arranged by applying an alternating current or direct current voltage to electrodes disposed on the inner or outer surface of the casting mold while giving an electromagnetic vibration, an impact force or impact mechanical vibration to the outside or interior of the casting mold, and polymerization is carried out in this state, and a process in which the casting mold is moved in predetermined directions while the liquid material is still in the flowable state. These processes, however, have problems in that the operation is batchwise, the equipment and economical costs are high, and the luster becomes uneven. As an epoch-making means for obtaining a sheet or plate having a uniform performance, a process was proposed in which, while the polymerizable liquid material is polymerized and solidified between confronting belts for the continuous production of a sheet or plate, an appropriate linear load is applied in the lateral direction of the belts (Japanese Unexamined Patent Publication No. 55-41,289).
However, these conventional processes intend only to obtain a pearl-like luster, and the light transmission of the product is low, and even if the light transmission is increased by reducing the amount of the leaf added, no substantial difference is found between the luminous transmittance and the solar radiation transmittance.
A light-diffusing plate comprising inorganic light-diffusing fine particles of barium sulfate, aluminum hydroxide, calcium carbonate or silicon dioxide, or fine particles of a transparent polymer having a refractive index different from that of the base resin, is disclosed, for example, in Japanese Unexamined Patent Publication No. 53-98,354. In the product obtained according to this process, there is no substantial difference between the luminous transmittance and the solar radiation transmittance, or the luminous transmittance is lower than the solar radiation transmittance. As a means for reducing the solar radiation transmittance below the visual transmittance, a process is known in which an alternating layer structure comprising a thin layer of a dielectric substance having a high refractive index and an optically multi-layered interference film is formed as the top layer on a substrate having a high transparency (Japanese Unexamined Patent Publications No. 51-118,451, No. 56-28,488, and No. 53-81,144). However, this process has problems in that, since vapor deposition or sputtering on the surface of a transparent plate is necessary, the manufacturing cost is high and the durability of the multi-layered film is unsatisfactory.